Collisional diffraction emerges from simple control of limbless locomotion

Abstract

Snakes can utilize obstacles to move through complex terrain, but the development of robots with similar capabilities is hindered by our understanding of how snakes manage the forces arising from interactions with heterogeneities. To discover principles of how and when to use potential obstacles, we studied a desert-dwelling snake, C. occipitalis, which uses a serpenoid template to move on homogeneous granular materials. We tested the snake in a model terrestrial terrain—a single row of vertical posts—and compared its performance with a robophysical model. Interaction with the post array resulted in reorientation of trajectories away from the initial heading. Combining trajectories from multiple trials revealed an emergent collisional diffraction pattern in the final heading. The pattern appears in both the living and robot snake. Furthermore, the pattern persisted when we changed the maximum torque output of the robot motors from 1.5 N-m to 0.38 N-m in which case local deformation of the robot from the serpenoid curve appears during interaction with the posts. This suggests the emergent collisional diffraction pattern is a general feature of these systems. We posit that open-loop control of the serpenoid template in sparse terrains is a simple and effective means to progress, but if adherence to a heading is desired more sophisticated control is needed.